Atrack coach wants to test the effectiveness of a new training program for distance runners. he selects his two fastest runners and, using a coin flip, randomly chooses one to participate in the new program. the other will continue with the standard training regimen. he then picks the next two fastest runners and randomly assigns one to each program. he repeats this process with all his runners, choosing the two fastest remaining each time. he then compares race times within each group of two runners. what is the name for this kind of experimental design?